The invention relates to a liquid measuring cell for measuring optical properties of liquids, in particular for NIR measurements, with a measuring chamber, which is adjoined on one side by a transparent window, and with a reflector disposed on the opposite side of the measuring chamber, which reflects the light striking it toward the outside through the measuring chamber and the window.
A liquid measuring cell of this type has been disclosed, for example, by DE 31 03 476 C2. The known liquid measuring cell is designated for the combined measurement of reflection and transmission properties (transflection) of liquids in the near infrared range NIR. To this end, it has a transparent measuring window on top through which the radiation of a radiation source enters a measuring chamber adjoining the measuring window, where it is partially reflected, partially scattered, and partially absorbed by the liquid. The portion of the radiation that passes through the liquid is diffusely reflected by a reflector, which has a rough gold surface and is disposed opposite the measuring window, and then passes through the liquid once more. The scattered and reflected portions then partially reemerge from the window. In the transmission, certain spectral ranges of the radiation are more intensely absorbed or scattered than others so that a spectral analysis of the diffuse light emerging from the measuring window permits conclusions to be drawn with regard to the type and quantity of constituents in the liquid tested.
In the known liquid measuring cell, the measuring chamber is adjoined on top by the measuring window and is adjoined on the bottom by the reflector. The surface of the reflector is therefore wet by the liquid. This device has the disadvantage that light-weight constituents of the liquid adhere to the rough surface of the reflector and change its optical properties. This severely impairs the reproducibility of the measurements, which has a particularly negative impact on series of measurements. This can in fact be remedied initially by frequently cleaning the reflector surface. But this is accompanied by a significant additional cost and does not produce satisfactory lasting results since frequent cleaning alters and finally destroys the optical properties of the rough surface. The reproducibility of the measurement results is in turn impaired as a result. Finally, when the surface coating is destroyed, the underlying material of the reflector as a rule suffers corrosion so that the entire reflector becomes unusable in the end.
The object of the current invention, therefore, is to disclose a liquid measuring cell of the type mentioned at the beginning, which is easy to clean and permits very reproducible measurements over long periods of time.
The object is attained according to the invention by virtue of the fact that another transparent window is disposed between the reflector and the measuring chamber and this additional window adjoins the measuring chamber on the other side.
With the device according to the invention, instead of coming into contact with the reflector surface, the liquid only comes in contact with the additional window. The window can be made of a smooth, easy-to-clean material, preferably glass, while the rough reflector surface is not wet and contaminated by the liquid. Therefore it does not need be cleaned and is thus also not subjected to wear. The additional window also sharply reduces the cost of cleaning and the possibly necessary replacement of the reflector and significantly improves the reproducibility of the measurement results. In this connection, this is surprising insofar as additional optical elements in the beam path fundamentally represent additional error sources which can also impair measurement results.
The uniformity of the layer thickness of the liquid, i.e. the thickness of the measuring chamber in the measuring region, is crucial to the reproducibility of the measurement results. In this connection, the device known from DE 31 03 476 C2 has the disadvantage that the O-ring seal disposed between the measuring window and reflector is elastic so that after the opening and closing of the measuring cell, the layer thickness is not reliably reproduced. Furthermore, slight deposits of the liquid form in particular on the surface of the O-ring, which must be expensively removed during cleaning.
Therefore, in order to improve the reproducibility of the measurement results and to make cleaning easier, a particularly simple embodiment of the invention is suggested in which the measuring chamber is embodied as a hollow space between a transparent bottom chamber part and a transparent top chamber part that rests on the bottom chamber part. There are no O-rings to become contaminated and the hard materials of the top chamber part and bottom chamber part that rest against each other continuously assure a precisely reproducible layer thickness in the measuring chamber. Since the measuring chamber is comprised of only two simply embodied components, it has practically no corners, edges, or recesses for impurities to become trapped in. It is thus particularly easy to clean.
A thorough cleaning is particularly simplified by means of the measure of embodying the top chamber part as removable because with the top chamber part removed, all of the inner surfaces that come into contact with the liquid are easily accessible for a mechanical cleaning.
In order to permit the manual removal of the top chamber part without special tools, it is suggested that the top chamber part be embodied in a cap-shaped fashion as a disk with an edge suitable for a screw or bayonet connection.
An embodiment that is simple and inexpensive to manufacture provides that the top chamber part is embodied as a plane-parallel disk, preferably a glass disk, whose underside rests against a flat surface of the bottom chamber part, and has a plane-parallel recess which, together with the surface of the bottom chamber part, defines the measuring chamber. The depth of the plane-parallel recess determines the layer thickness of the liquid to be tested and also essentially determines the volume of the measuring chamber.
In order to permit a simple and rapid change of the layer thickness, it is suggested that a number of interchangeable top chamber parts be provided, which have plane-parallel recesses with different depths.
The inflow and outflow of the liquids to be tested, during and after the measurements, requires liquid lines which lead into the measuring chamber at suitable locations. A simple embodiment of the liquid inlet and outlet provides that the bottom chamber part is provided with two conduits for the inflow and outflow of the liquid to be tested, which lead from the underside of the bottom chamber part to a liquid inlet and a liquid outlet of the measuring chamber. Then the continuing liquid lines can easily be connected to the conduits from the outside.
The reproducibility of the measurement results can be further improved by virtue of the fact that the bottom chamber part has an annular recess disposed in the edge region of the plane-parallel recess of the top chamber part and the conduits feed into this annular recess from radially opposing points. This measure assures that all the way around the measuring region, a slightly thicker bypass for the liquid is produced in which air bubbles possibly contained in the liquid, which could distort the measurement results, are conveyed around the measurement region.
In order to position the reflector as close to the measuring chamber as possible, the provision is made that the bottom chamber part has a central, cylindrical reflector bore which is let into it from its underside and which has the reflector disposed in it, where a preferably plane-parallel window region of the bottom chamber part between the reflector and the measuring chamber constitutes the additional transparent window. This window is relatively thin in comparison to the remaining thickness of the bottom chamber part due to the reflector bore that has been let into it so that an unfavorable optical absorption of the window is reduced and the heat transmission between the reflector and the measuring chamber is improved.
In a simple embodiment, the provision is made that the reflector is comprised of a reflector body with an essentially cylindrical top part whose end has a reflective layer affixed to it.
In order to keep the characteristics of the diffusely reflected radiation as true as possible, it is preferable that the reflective layer be comprised of a rough gold coating.
In a preferred embodiment, the reflector body is comprised of a favorably heat conductive material, preferably copper, and can be heated and/or cooled. This permits the temperature of the test liquid to be kept constant and thereby contributes to the improvement in the reproducibility of the measurement results which are very temperature dependent. Furthermore, the temperature can be kept constant with a very low expenditure of energy because the reflector body has a relatively low volume/mass ratio and is placed close to the measuring chamber. As a result, only a small amount of heat escapes unused into the environment or (during cooling operation) comes in from the environment. Since only smaller masses have to be heated/cooled, the adjusting time for a new temperature is very short.
In a modification of the invention, a Peltier element is provided as a heating and/or cooling element. The Peltier element only requires current and does not require an expensive water connection and a changeover from heating to cooling can be produced by simply reversing the current direction.
The measure that the reflector rests with its end against the window region of the bottom chamber part with an elastic initial stress, assures a good thermal contact with the measuring chamber and constant conditions in the optical transition between the window region of the bottom chamber part and the reflector, in particular, a plane-parallel contact without excessive pressure.
In order to prevent the distortion of measurement results due to mechanical stresses and flections of the optically active elements, it is suggested that a heat conducting intermediary piece connected to the heating element be provided to compensate for longitudinal expansion at different temperatures, and this intermediary piece has a preferably annular groove on top into which a preferably tubular bottom part of the reflector body is movably slid.
The heat transmission of this device can be still further improved by virtue of the fact that the groove is provided with a favorably heat conductive, permanently pasty material, preferably with heat conducting paste.
The initial stress required to press the reflector against the bottom chamber part can be produced simply in that a compensation disk or disk spring is disposed at the groove bottom in order to produce an elastic initial stress between the reflector body and the intermediary piece.
In order to improve the temperature constancy of the liquid sample, in the window region, the bottom chamber part has a temperature sensor which is preferably disposed eccentrically in the vicinity of the liquid inlet. This temperature sensor can cooperate with the heating/cooling element in a generally known control circuit in order to automatically stabilize the temperature of the liquid sample in relation to different environmental influences and in order to set a predetermined temperature from a designated range.
In order to largely eliminate interfering environmental influences and reduce the reaction time when adjusting and maintaining temperatures, it is suggested that in the window region, the bottom chamber part have a recess leading from the reflector bore, in which the temperature sensor is accommodated so that it is situated close to the surface of the bottom chamber part that adjoins the measuring window. The proximity to the liquid sample largely permits the temperature sensor to determine the true sample temperature without significant delay.